Describing the structural geology and geologic history of the west central Conger Range, Utah
Timothy Nixon
The Basin and Range is a geologically complex area that has experienced both compressional and extensional strain. The compression was a result of the Cretaceous (100-70Ma) Sevier Orogeny, during which thrusting created large (tens of square kilometers) decollemonte surfaces and folds. The present topography is dominated by the middle to late Tertiary extension which extended the crust ~100 %. The extension created large, scale low angle surfaces that mimic the geometry of the Cretaceous thrust detachments.
The Conger Range, a part of the Basin and Range province in western Utah show these characteristics. The goal of my research was to explain the geometric problem exhibited in the Mississipian Joana Limestone. This formation is 200 ft thick (60 m) and dipping to the east an average of 30° in the study area, yet is exposed on the surface for nearly 3/4 mile (~1 km). My mapping shows that imbricate, high angle, normal faults have broken up the formation to its present geometry. Drag folding along the footwall blocks of these west dipping faults have the structures that indicate normal movement along the fault planes or zones. Offset seen in cross-section indicates that the fault angle shallows at depth, and that these faults are therefore listric.
Another anomaly that is interesting is the sudden change in dip in the northern region of the research area, from east to north. This was caused during the Sevier orogeny when a thrust sheet composed of mainly Mississippian to Devonian age rock was broken up from differential thrusting over the footwall block. This caused the thrust sheet to have seperate levels of detachment surfaces. The northern dip in the north part of my research area is the stratigraphy accommodating this difference of detachment surfaces, thus creating a north dipping backlimb of a thrust sheet. There are also many east-west trending normal faults with the north hanging walls down-dropped in this area, again to accommodate this change in structural level.
Figure 1. Plate 2 (A-A') shows the main transect across my research area, showing how the Mississippian Joana Limestone (Mj) was broken up by listric normal faults, as well as the structure found in the Devonian Guilmette Limestone. At 1400 ft above sea level the main detachment surface is found. This surface was created during the Cretaceous Sevier orogeny, and was reactivated in Tertiary times as an extensional surface when the continental crust is to have extended on the order of 100%.
